The process of creating a computer chip consists of manufacturing a wafer which then goes through the basic operations of layering, patterning, doping, and heat treatment. This invention discloses a method of layering (i.e., metallization) in preparation for the patterning process.
One of the layers in the layering process consists of a metallic film layer which will eventually later become conductor, or wires between chip elements, e.g., source, drain, and gate in the case of MOS devices. The materials, methods, and processes of "wiring" the component parts together is generally referred to as metallization. The metallization process starts in the masking area where small holes, called contact holes or vias, are etched through all of the surface layers, down to the active regions of the devices. Following contact masking, a thin layer (10,000 to 15,000 .ANG.) of the conducting metal is deposited by vacuum evaporation, sputtering, or chemical vapor deposition (CVD) techniques over the entire wafer.
In the role of surface conductor, a metal must meet the following criteria:
good electrical currency-carrying capability; PA1 good adhesion to the top surface of the wafer; PA1 ease of patterning; PA1 good electrical contact with the wafer material; PA1 high purity; PA1 corrosion resistance; and, finally, in the case of VLSI integrated circuits, PA1 low reflectivity.
Aluminum or an aluminum alloy is most often used as the metal interconnect in semiconductor manufacture. After deposition, a pulsed laser is generally utilized for further processing the metallic layer, such as to melt and evenly distribute this metallic layer on the substrate. Laser processing may be used, for example, to solidly fill small contact vias on the substrate of an (IC) wafer and to induce the flow of molten metal interconnect into the contact vias. The laser melting of the metal film fills these micron sized windows while preserving various devices located on the (IC) wafer.
In use of laser processing, it often is desirable to apply enough laser energy to heat a metallic layer sufficiently to flow into the vias but not so much as to cause ablation or material loss of the metallic film. Optimal high and low laser energy limits, or a process window, is thus often present. The process of the invention is directed towards widening this process window by lowering the optical reflectivity and increasing the optical absorptivity of a metallic layer during laser processing.
Laser processing of a metallic layer is shown in FIG. 1. With reference to FIG. 1, a semiconductor wafer 12 includes a (Si) substrate 16 having various active regions 14 and an oxide layer (SiO.sub.2) 26. A metallic layer 20, such as (Al), is deposited over the oxide layer (SiO.sub.2) 26 and into vias 22 for contacting the active regions 14 formed on the wafer substrate (Si) 16. As noted by the dotted lines in the (Al) layer 18, the (Al) layer 18 is initially deposited with poor step coverage over the via 22. The laser processing (i.e., laser reflow) melts and planarizes the (Al) layer 20 so that it completely fills the contact via 22.
During laser processing, a high-intensity laser light beam 10 is directed to the surface 18 of the metallic layer 20 for planarizing and melting the metallic layer 20 to provide a good electrical contact to the active regions 14. The laser beam 10 is ideally directed at a 90.degree. angle to the surface 18. When this ideal situation exists, a minimum of the incident laser beam 10 reflects directly up from the metallic surface 18. In reality, however, some of the incident beam 10 is traveling at angles other than 90.degree. to the metallic surface 18 and is reflected away from the metallic surface 18 as indicated by arrows 24. This reflectivity necessitates higher laser fluences and increased laser process times.
This surface reflectivity of metal layer 20 varies with the material and the surface smoothness of the metallic surface 18. Metal layers, however, especially aluminum and aluminum alloys, have relatively high optical reflectivity properties. A goal of the process of the invention is the formation of a surface to control this form of reflection.
Prior art processes to reduce optical reflectivity on these metal surfaces have included the use of antireflective coatings. Antireflective coatings (ARCs) are generally sputtered onto the wafer metallic surface before laser planarization to reduce optical reflectivity of the laser pulse and, therefore, retain more energy within the metallic layer to improve partial melting. As an example, a prior art ARC deposition may consist of sputtering a 100 .ANG. metallic film such as titanium, a titanium and tungsten alloy, or an amorphous silicon layer on the surface 18 of the metallic layer 20, FIG. 1, prior to the laser processing.
There are several problems associated with the use of an ARC. One is that the additional layer of ARC material requires a separate deposition process, such as sputtering. In addition, the time of laser exposure for the ARC layer can increase 30 to 50 percent, increasing the wafer throughput time. Additionally, some of the ARC material can become alloyed with the metal layer during melting causing an increase in resistivity. Moreover, some ARCs may cause photoresist adhesion problems.
It is therefore, the purpose of the invention to improve the laser reflow process by reducing optical reflectivity and increasing optical absorptivity of a metal layer using a chemical mechanical planarization process. This overcomes the drawbacks associated with the use of prior art ARCs and dyes.